Mammalian telomerase is a specialized reverse transcriptase that extends the 3' ends of chromosomes with telomeric DNA. Because telomerase is weakly expressed in somatic cells, but is overexpressed in 90% of cancer cells, it serves as an attractive target for anti-cancer drug design. A multi- protein complex known as shelterin associates specifically with telomeric DNA to repress illicit DNA fusions at mammalian chromosome ends. If the normal function of shelterin is to 'protect' chromosome ends, how does telomerase gain access to these ends to extend them? POT1-TPP1 is a shelterin sub-complex that binds single-stranded telomeric DNA with high specificity and affinity. A major function of POT1-TPP1 in vivo is to repress DNA damage recognition events at telomeres. Given its role in chromosome end-protection, POT1- TPP1 might be expected to inhibit telomerase by preventing its access to chromosome ends. Surprisingly, POT1-TPP1 increases telomerase processivity in vitro. Additionally, the OB domain of TPP1 is involved in telomerase recruitment to telomeres. The stimulation of telomerase by POT1-TPP1 has critical physiological significance insofar as the telomerase activity associated with cancer cells might require POT1-TPP1-based stimulation. Here, it is hypothesized that a surface on the OB domain of TPP1 interacts directly with telomerase to give rise to telomerase recruitment and stimulation. Enzymology in combination with mammalian cell biology and structural biology will be used to test this hypothesis and determine the consequence of telomerase stimulation by TPP1 in cancer cells. This will be the first study to assess directly the biological importance of telomerase stimulation by TPP1 or any mammalian shelterin subcomplex. The specific aims of the project are to: 1. Identify structural elements in human TPP1 that lead to telomerase processivity stimulation using a site-directed mutagenesis screen, looking for separation-of-function mutants defective specifically in telomerase stimulation in vitro but not in DNA end-protection. 2. Determine the physiological importance and mechanism of telomerase stimulation by TPP1 in HeLa-based and lung cancer cell-lines that knock down endogenous TPP1 and express wild-type or telomerase stimulation-defective mutants of TPP1 in a stable manner. These cell lines will be tested for telomere length defects and telomerase recruitment defects. 3a. Obtain a high-resolution view of chromosome-end protection by POT1-TPP1 by crystallizing a POT1-TPP1 fusion protein in complex with telomeric DNA. 3b. Obtain insights into telomerase stimulation by TPP1 by crystallizing a biochemically competent, minimal, TPP1-telomerase complex defined through truncation analyses of the individual components. The K99 phase of the proposed aims will be conducted under the mentorship of Dr. Tom Cech, whose mentoring skills have helped more than 30 of his mentees to attain faculty positions in research institutions in the US and worldwide. The Cech lab is a leader in the biochemistry of telomerase and telomeres, and is equipped with the resources required to address the biochemical/structural aims of the proposed study. For the HeLa-based experiments, we have an ongoing collaboration with Dr. Leslie Leinwand of the Mol. Cell and Dev. Biology Department of UC Boulder. For the experiments in the lung cancer cell lines, I will be co- mentored by Dr. James DeGregori of the UC Cancer Center Denver and will have full access to the facilities of his lab and the Cancer Center. Hence, I strongly believe that the facilities at CU Boulder and at the Cancer Center will provide me with the ideal environment to execute the proposed goals of the K99/R00 application. My goal in the K99 phase of research is to complete, in 2 years, Aim 1 and Aim 2A&C of the proposal, and apply for an independent faculty position in the US. Aims 2B&D and Aim 3 will be completed in the R00 phase. In the long-term, I wish to become an independent investigator, running a lab consisting of people from various backgrounds (biochemistry, structural biology, and cell biology) working together to answer critical questions in telomere biology and its implications in cancer. In addition to allowing me to hire staff and bu lab supplies, the K99/R00 award will greatly facilitate my postdoc-to-PI transition by allowing me to attend a cancer biology course, a telomerase-cancer AACR meeting, and a microscopy workshop conducted by Cold Spring Harbor labs. I have obtained formal training in the responsible conduct of research (RCR) during my Ph.D. and will continue to take steps to acquire RCR training during and after my postdoc. I began my research career as an M.S. student synthesizing small molecules, but have since shifted my focus to more to bio-oriented problems. As a technician I studied protein folding, then, as a graduate student I employed biochemistry and x-ray crystallography to study RNA/DNA repair, and now, working as a post-doc with Dr. Tom Cech I am beginning to study telomerase regulation in human cancer cells. During the course of my scientific training, I have learnt theoretical concepts and developed experimental skills in diverse areas of research. I believe that the knowledge and experience I have gained thus far will greatly assist in the successful completion of the aims of the K99/R00 proposal in a timely fashion. PUBLIC HEALTH RELEVANCE: Telomerase is an enzyme that is overproduced in 90% of human cancers and is therefore considered a major drug target for fighting cancer. Biochemically, this enzyme is stimulated by a protein called TPP1, and the importance of this stimulation in cancer cells will be tested in the proposed study. From this study, it will be possible to understand how TPP1 stimulates telomerase to full potency in cancer cells, and the knowledge gained may pave the path to the design of a new generation of anti-cancer drugs.